diff --git a/.github/workflows/html5lib-tests.yml b/.github/workflows/html5lib-tests.yml
new file mode 100644
index 0000000000000..9ff0b1aae0873
--- /dev/null
+++ b/.github/workflows/html5lib-tests.yml
@@ -0,0 +1,195 @@
+name: HTML API html5lib Tests
+
+on:
+  push:
+    branches:
+      - trunk
+      # The html5lib tests were introduced in WordPress 6.6.
+      - '6.[6-9]'
+      - '[7-9].[0-9]'
+      - '[1-9][0-9]+.[0-9]'
+    tags:
+      - '6.[6-9]'
+      - '6.[6-9].[0-9]+'
+      - '[7-9].[0-9]'
+      - '[7-9].[0-9].[0-9]+'
+      - '[1-9][0-9]+.[0-9]'
+      - '[1-9][0-9]+.[0-9].[0-9]+'
+  pull_request:
+    branches:
+      - trunk
+      - '6.[6-9]'
+      - '[7-9].[0-9]'
+      - '[1-9][0-9]+.[0-9]'
+    paths:
+      # Any change to a PHP file in the HTML API should run checks.
+      - 'src/wp-includes/html-api/**.php'
+      - 'tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php'
+      - 'tests/phpunit/data/html5lib-tests/**.dat'
+  workflow_dispatch:
+
+# Cancels all previous workflow runs for pull requests that have not completed.
+concurrency:
+  # The concurrency group contains the workflow name and the branch name for pull requests
+  # or the commit hash for any other events.
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  cancel-in-progress: true
+
+# Disable permissions for all available scopes by default.
+# Any needed permissions should be configured at the job level.
+permissions: {}
+
+jobs:
+  # Runs the HTML API html5lib PHPUnit tests for WordPress.
+  #
+  # Performs the following steps:
+  # - Sets environment variables.
+  # - Checks out the repository.
+  # - Sets up Node.js.
+  # - Sets up PHP.
+  # - Installs Composer dependencies.
+  # - Installs npm dependencies
+  # - Logs general debug information about the runner.
+  # - Logs Docker debug information (about the Docker installation within the runner).
+  # - Starts the WordPress Docker container.
+  # - Logs the running Docker containers.
+  # - Logs debug information about what's installed within the WordPress Docker containers.
+  # - Install WordPress within the Docker container.
+  # - Run the HTML API html5lib PHPUnit tests.
+  # - Ensures version-controlled files are not modified or deleted.
+  run-html-api-html5lib-tests:
+    name: PHP ${{ matrix.php }}
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    if: ${{ github.repository == 'WordPress/wordpress-develop' || github.event_name == 'pull_request' }}
+    strategy:
+      fail-fast: false
+      matrix:
+        php: [ '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3' ]
+
+    steps:
+      - name: Configure environment variables
+        run: |
+          echo "PHP_FPM_UID=$(id -u)" >> $GITHUB_ENV
+          echo "PHP_FPM_GID=$(id -g)" >> $GITHUB_ENV
+
+      - name: Checkout repository
+        uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+        with:
+          show-progress: ${{ runner.debug == '1' && 'true' || 'false' }}
+
+      - name: Set up Node.js
+        uses: actions/setup-node@60edb5dd545a775178f52524783378180af0d1f8 # v4.0.2
+        with:
+          node-version-file: '.nvmrc'
+          cache: npm
+
+      ##
+      # This allows Composer dependencies to be installed using a single step.
+      #
+      # Since the tests are currently run within the Docker containers where the PHP version varies,
+      # the same PHP version needs to be configured for the action runner machine so that the correct
+      # dependency versions are installed and cached.
+      ##
+      - name: Set up PHP
+        uses: shivammathur/setup-php@a4e22b60bbb9c1021113f2860347b0759f66fe5d # v2.30.0
+        with:
+          php-version: '${{ matrix.php }}'
+          coverage: none
+
+      # Since Composer dependencies are installed using `composer update` and no lock file is in version control,
+      # passing a custom cache suffix ensures that the cache is flushed at least once per week.
+      - name: Install Composer dependencies
+        uses: ramsey/composer-install@57532f8be5bda426838819c5ee9afb8af389d51a # v3.0.0
+        with:
+          custom-cache-suffix: $(/bin/date -u --date='last Mon' "+%F")
+
+      - name: Install npm dependencies
+        run: npm ci
+
+      - name: General debug information
+        run: |
+          npm --version
+          node --version
+          curl --version
+          git --version
+          composer --version
+          locale -a
+
+      - name: Docker debug information
+        run: |
+          docker -v
+
+      - name: Start Docker environment
+        run: |
+          npm run env:start
+
+      - name: Log running Docker containers
+        run: docker ps -a
+
+      - name: WordPress Docker container debug information
+        run: |
+          docker compose run --rm mysql ${{ env.LOCAL_DB_TYPE }} --version
+          docker compose run --rm php php --version
+          docker compose run --rm php php -m
+          docker compose run --rm php php -i
+          docker compose run --rm php locale -a
+
+      - name: Install WordPress
+        run: npm run env:install
+
+      - name: Run HTML API html5lib PHPUnit tests
+        run: node ./tools/local-env/scripts/docker.js run php ./vendor/bin/phpunit --verbose -c phpunit.xml.dist --group html-api-html5lib-tests
+
+      - name: Ensure version-controlled files are not modified or deleted
+        run: git diff --exit-code
+
+  slack-notifications:
+    name: Slack Notifications
+    uses: WordPress/wordpress-develop/.github/workflows/slack-notifications.yml@trunk
+    permissions:
+      actions: read
+      contents: read
+    needs: [ run-html-api-html5lib-tests ]
+    if: ${{ github.repository == 'WordPress/wordpress-develop' && github.event_name != 'pull_request' && always() }}
+    with:
+      calling_status: ${{ contains( needs.*.result, 'cancelled' ) && 'cancelled' || contains( needs.*.result, 'failure' ) && 'failure' || 'success' }}
+    secrets:
+      SLACK_GHA_SUCCESS_WEBHOOK: ${{ secrets.SLACK_GHA_SUCCESS_WEBHOOK }}
+      SLACK_GHA_CANCELLED_WEBHOOK: ${{ secrets.SLACK_GHA_CANCELLED_WEBHOOK }}
+      SLACK_GHA_FIXED_WEBHOOK: ${{ secrets.SLACK_GHA_FIXED_WEBHOOK }}
+      SLACK_GHA_FAILURE_WEBHOOK: ${{ secrets.SLACK_GHA_FAILURE_WEBHOOK }}
+
+  failed-workflow:
+    name: Failed workflow tasks
+    runs-on: ubuntu-latest
+    permissions:
+      actions: write
+    needs: [ slack-notifications ]
+    if: |
+      always() &&
+      github.repository == 'WordPress/wordpress-develop' &&
+      github.event_name != 'pull_request' &&
+      github.run_attempt < 2 &&
+      (
+        contains( needs.*.result, 'cancelled' ) ||
+        contains( needs.*.result, 'failure' )
+      )
+
+    steps:
+      - name: Dispatch workflow run
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          retries: 2
+          retry-exempt-status-codes: 418
+          script: |
+            github.rest.actions.createWorkflowDispatch({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              workflow_id: 'failed-workflow.yml',
+              ref: 'trunk',
+              inputs: {
+                run_id: '${{ github.run_id }}'
+              }
+            });
diff --git a/tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php b/tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php
index c40481ac18e45..a43c48851af63 100644
--- a/tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php
+++ b/tests/phpunit/tests/html-api/wpHtmlProcessorHtml5lib.php
@@ -12,6 +12,8 @@
  *
  * See the README file at DIR_TESTDATA / html5lib-tests for details on the third-party suite.
  *
+ * A change to test that the workflow triggers.
+ *
  * @package WordPress
  * @subpackage HTML-API
  *